An image sensor is a semiconductor device for converting an optical image into an electrical signal. The image sensor is generally classified as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
A unit pixel of the CIS typically includes a photodiode and a metal oxide semiconductor (MOS) transistor. In operation, the CIS sequentially detects an electrical signal of the unit pixel in a switching manner to generate an image.
To improve the optical sensitivity of the image sensor, a method of increasing the fill factor of the image sensor or concentrating light incident to other regions except for the photodiode onto the photodiode is used.
One such method for concentrating light involves forming a microlens.
According to a related art, the process for forming a microlens in a manufacturing method for an image sensor typically includes a micro photo process using a photoresist for a microlens followed by a reflowing process to create convex shaped microlenses.
However, when reflowing the photoresist, an amount of the photoresist removed, generating gaps between microlenses. Thus, the amount of light incident to the photodiode decreases, causing a defective image.
Also, since corners of the microlenses (such as at an intersection of adjacent portions of four lenses) are rounded by various causes, the gaps occur between the adjacent microlenses.
FIGS. 1 and 2 are images illustrating photoresist patterns 50a for forming microlenses and the resulting microlenses 50b formed by reflowing the photoresist patterns 50a by heat according to a related art.
Light incident between rounded corners of the microlenses 50b may disappear, thereby degrading the sensitivity of an image sensor. When the four microlenses 50b are arranged in a matrix shape, a large opening occurs between the microlenses 50b. Thus, light penetrating through the opening may not be properly incident to the photodiode, considerably degrade the quality of an image.